Washington (CNN) -- Congress is launching a broad-ranging investigation into possible waste, misuse and corruption tied to billions of taxpayer dollars used to support private military contractors in Afghanistan.

Among the questions being raised is whether money provided in a nearly $2.2 billion trucking contract in the war-torn country went to pay off local warlords and the Taliban.

The investigation comes as the Obama administration begins deploying another 30,000 troops to Afghanistan. The White House has requested more than $70 billion to fund the war in fiscal year 2010, according to a Senate subcommittee memo.

"Serious allegations have been brought to [Congress'] attention that private security providers for U.S. transportation contractors in Afghanistan are regularly paying local warlords and the Taliban for security," said Rep. John Tierney, D-Massachusetts, who chairs the House National Security and Foreign Affairs Subcommittee.

"After a preliminary inquiry, it has been determined these reports warrant a full-scale ... investigation. If shown to be true, it would mean that the United States is unintentionally engaged in a vast protection racket and, as such, may be indirectly funding the very insurgents we are trying to fight."

Eight companies currently split the Afghan host nation trucking contract, which went into effect in May. The contract, according to Tierney's subcommittee, provides ground transportation in Afghanistan for more than 90 percent of supplies for U.S. troops, including food, fuel and ammunition.

Most supplies are shipped overland via Pakistan to Bagram Airfield, the main U.S. base in Afghanistan. They are then distributed to hundreds of other smaller airfields and bases.

In the Senate, an oversight subcommittee headed by Sen. Claire McCaskill, D-Missouri, also is looking into how contractor money is being spent.

Federal auditors state that approximately $950 million in "questioned and unsupported costs" has been submitted by Pentagon contractors for work in Afghanistan, according to a report prepared for Congress. That amount represents 16 percent of the total contract dollars examined so far.

The U.S. government has spent more than $23 billion on contracts in Afghanistan since 2002, the report states.

There are currently 104,000 Pentagon contractors in Afghanistan -- a figure that could rise to 160,000 to support President Obama's planned troop increase, according to the report. Roughly 100,000 U.S. troops are slated to be in Afghanistan at the height of the coming surge.